FIG. 1A shows a prior art communications system, which includes a System on a Chip (SoC) 110, typically comprising baseband processing for a wireless communications system, which is coupled to an RF Front End 112 which accepts signals from antenna 113, and performs the sequential operations of RF amplification, mixing to baseband using a local oscillator, and conversion to digital sampled signals using an analog to digital converter (ADC), and delivering these signals to the SoC 110 over interface 111. A transmit stream may be generated by SoC 110, which is provided to RF front end 112 as a baseband digital signal, which the RF front end 112 converts to an analog signal using a digital to analog converter (DAC). Thereafter, the analog signal is mixed to a modulation frequency, amplified, and transmitted to antenna 113. The SoC 110 accepts a clock signal NET_CLK generated by an accurate network clock source 106. Generating the baseband modulation signals requires a relatively accurate clock compared to the other operations of the SoC 110, and a low frequency SLEEP_CLK is sourced by a sleep clock generator 104. The SLEEP_CLK may be coupled to a power sequencer 114 such as for powering up the SOC 110 and RF front end 112 when periodic beacons are received. The SoC 110 may also receive a HOST_CLK from a host clock generator 108, which is also coupled to an applications processor 102 through host interface 116, which may be a synchronous interface according to a known standard such as Peripheral Component Interconnect (PCI as described in www.pcisig.com), Universal Serial Bus (USB as described in www.usb.org), Secure Digital IO (SDIO as described in www.sdcard.org), or any host interface known for interconnecting an applications processor to a communications system through an interface.
For battery powered devices, power saving modes are related to the useful time the device may be operated on a single charge. One prior art power saving mode uses a power sequencer 114, which powers down various components of the system, which is shown as separated into components related to transmitting and receiving wireless signals PD2 such as associated with the baseband interface 111 of the SoC 110. For example, if there is no anticipated activity on baseband interface 111, PD2 may be asserted, thereby putting RF front end 112 into a powerdown state when no transmit or receive activity is anticipated, and PD1 may be asserted when there is no anticipated data across the host interface 116. The assertion of partial powerdown for power-consuming parts of a processing element is known as a “sleep mode”, and may involve operation at a lower clock rate, or partial or complete powerdown of the associated system. Crystal oscillators such as those used to generate the host clock 108 and network clock 106 tend to consume a large amount of power compared to low frequency sleep clock 104, in part because the displacement currents generated by each clock transition in the oscillator as well as the circuitry the clock is delivered to are proportional to clock rate, such that for all other considerations being equal, a lowest rate clock tends to result in a lower power dissipation.
One problem of power saving operations is the requirement for the SoC 110 to maintain any existing network connections, and create new connections as required, both operations which require the SoC 110 to come out of sleep mode periodically and check for any pending traffic to be received or transmitted before going back into a sleep mode, and to be able to do this in a manner which does not cause any network connections or requests to time out for failure to respond. In one prior art system, the sleep clock 104 operates a wake-up timer within power sequencer 114, such that the SoC 110 and RF front end 112 are powered up to receive periodically transmitted signals such as beacons, and any required transmit frames are sent during these intervals.
Outside of such wake-up intervals, if the communication SoC 110 does not have a clock, it will not be able to serve incoming requests 103 from the application processor 102. In one prior art system, the SoC 110 indicates to the application processor 102 that it is entering a sleep mode, and the applications processor 102 uses a wakeup protocol with sequencer 114 to bring the system out of sleep mode when making a request 103. In this system, the application processor 102 will queue requests and assert a powerup request to sequencer 114. When the SOC 110 comes out of sleep mode and has clock signals available, it indicates to the application processor 102 through a handshake mechanism across interface 116 that the application processor 102 may start sending requests and other relevant events.
FIG. 1B shows the timing associated with this prior art wake-up method. Until request time 152, only sleep clock 168 is active, and the host clock 164 and network clock 170 are powered down. After host request 152, HOST_CLK 164 is in a shutdown state until Wakeup SoC is asserted 154, where the HOST_CLK stabilizes during an initialization time, and at time 156, the HOST_CLK is stabilized and the request is handled, with the network clock 170 applied thereafter 156. After the network events are handled from time 156 to time 158, host clock 164 enters a shutdown mode at time 158. The network clock 170 may stay active after end of request handling at time 158 to time 160 to complete the processing of any transmit network traffic which is generated, and enters a sleep mode thereafter 160.
There are many drawbacks associated with the process of FIGS. 1A and 1B. The latency in response from time 152 to time 154 followed by initialization until time 156 consumes additional time, during which interval the SOC has to be in a wake up mode prior to handling any actual requests, which also represents a power consumption inefficiency. The latency from time 152 to time 156 also results in reduced throughput if there are many such requests handled sequentially. Another inefficiency is that the applications processor 202 buffers the pending events without any of them being handled until the wake-up process from time 152 to time 156 is completed. Additionally, certain protocols such as Voice Over IP (VoIP) require immediate handling without the latency associated with wake-up protocols.